In the semiconductor art it is often desirable to form dielectric layers or films on various supporting structures, such as the gate insulator in field effect transistors, an insulator or passivation layer covering various areas (e.g. the extrinsic base region) of other types of transistors, such as HBTs and the like, an insulator or passivation layer surrounding the mesa or walls of a vertical cavity surface emitting laser or edge emitting lasers, etc. Regardless of the use, it is generally imperative that the dielectric layer or film be a good insulator with low defect density to enable device operation and enhance/maintain device performance. Also, the thickness of the layer must be sufficient to provide the required characteristics of the semiconductor devices, e.g. leakage current, reliability, etc.
Due to a lack of insulating layers on gallium arsenide (GaAs) based semiconductors providing low interface state density and stable device operation, the performance, integration level and marketability of both digital and analog GaAs based devices and circuits is significantly limited. As is known in the art, growing oxide films by oxidizing GaAs based materials results in high interface state density and a Fermi level which is pinned at the GaAs-oxide interface.
A method of forming a thin film of Ga.sub.2 O.sub.3 is disclosed in U.S. Pat. No. 5,451,548, entitled "Electron Beam Deposition of Gallium Oxide Thin Films Using A Single Purity Crystal Source", issued Sep. 19, 1995. Basically, this method consists of forming the thin film of Ga.sub.2 O.sub.3 from the electron beam evaporation of a single crystal of Gd.sub.3 Ga.sub.5 O.sub.12. The Gd.sub.3 Ga.sub.5 O.sub.12 decomposes into Gd.sub.2 O.sub.3 (gadolinium oxide) and Ga.sub.2 O.sub.3 (gallium oxide) during evaporation. The major problem is that some of the Gd.sub.2 O.sub.3 (about 0.1% according to the patent) is incorporated into the thin film as impurities creating defects and increasing the bulk trap density. The increased defect and, in turn increased bulk trap density degrades the performance of the devices in which the thin film is used. Further, the density of impurities significantly increases as the thickness of the thin film is increased.
Accordingly, it would be highly desirable to provide a method of fabricating dielectric layer structures with significantly reduced defect densities and, consequently, reduced bulk trap densities.
It is a purpose of the present invention to provide a new and improved method of fabricating dielectric layer structures.
It is another purpose of the present invention to provide a new and improved method of fabricating dielectric layer structures having substantially reduced defect and bulk trap densities.
It is a further purpose of the present invention to provide a new and improved method of fabricating dielectric layer structures which result in stable semiconductor device performance.